01. Exit Statuses

When a program finishes, it returns a integer value. This code is known as the exit status or exit code. Depending on its value, we can see if the program terminated successfully or not.

If the value is 0, that means the program ran without a problem (this is why you return 0 in C programs). However, any value of 1 or greater means that it terminated unsuccessfully.

So...what do exit statuses have to do with control flow? Great question! We'll how all this ties in together real soon when we learn if-else statements.

Last exit status $?

To find the last exit status, simply use the $? variable.

Be cautious in that if you run echo $? twice in a row, the second command will always return a 0. Can you guess why?

Let's try some examples.

$ rm /; echo $?
rm: /: is a directory
$ ls -l; echo $?
# Lists folder contents
$ echo $?
# Running 'echo $?' twice always produces a success

POSIX exit statueses

Here is a list of POSIX exit statuses.

> 0
Failure due to redirection or word expansion.
Unsuccessful (for the most part, but depends on command).
File not executable.
Command not found.
Unspecified, but failure.
Command failed due to receiving a signal.
$ ls -l hello_world
# Not executable
-rw-r--r--  1 johnPC  staff  0 Jul  3 09:41 hello_world
$ ./hello_world
-bash: ./hello_world: Permission denied
$ echo $?
$ asdf
$ echo $?

Passing an exit value per script

To terminate your script with a specific exit code, use the exit code, followed by an integer.

if [ ! -e $SAMP_FILE ]; then
    exit 32

We'll learn if statements real soon, but this snippet simply checks if the file does not exist.

Exceptions to the 0 successful rule

In some cases, a program can exit with a value other than 0, and still be considered a successful run.

For example, the grep command returns a 0 if a matching pattern is found, and a 1 if no matching pattern is found. A value of 2 or greater is reported for real errors.

To ensure of no bugs in your code, make sure to check out a command's EXIT VALUE or DIAGNOSTICS section of its man page!

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